1. Field of the Invention
The present invention relates to the detection of dental disease using polarized light by optically measuring the depolarization of incident light backscattered from dental tissues.
2. Description of Related Art
Dental caries, or tooth decay, is a pathological process of destruction of tooth structure by oral microorganisms, which can lead to tooth loss if untreated. In coronal caries, lesions begin in the enamel and cause demineralization of the enamel. This demineralization changes the scattering properties of the enamel, resulting in chalky or xe2x80x9cwhite spotxe2x80x9d lesions visible when the caries occurs on smooth, unstained enamel surfaces. If the carious lesion is detected before it reaches the dentin, remineralization is still possible. After the carious lesion has reached dentin, however, inflammation of the pulp occurs, requiring a filling and leading to serious tooth decay and eventual tooth loss if untreated. Restorative dentistry is most effective when the progression of caries is detected early before it reaches the dentin.
Current techniques for diagnosing caries are visual inspection, mechanical probing with a sharp dental explorer, and radiographic imaging. The tooth can be tactilely and visually explored to determine the presence of indicators of tooth decay such as surface irregularities, crevices, or discoloration. However, the practice of probing all accessible tooth surfaces with a sharp explorer is coming under increased scrutiny since it can further damage enamel already weakened by decay and may also cause cross-contamination between teeth. As tooth decay primarily affects the region of calcium below the tooth surface, detection of caries before significant damage occurs in the tooth is very difficult.
By the time caries is evident under visual and tactile examination of the tooth, the disease is usually in an advanced stage, requiring a filling and occasionally leading to tooth loss. As a consequence of conservative diagnoses and treatment, there are false positives leading to unnecessary drilling and placement of restorations in healthy teeth. Currently there is no accurate device for determining whether restorations are in need of replacement, resulting in enormous costs from the unnecessary replacement of good restorations and complications such as root canals from not replacing defective or aged restorations.
Radiography is often used for detection of cavities, since it provides integrated views of tooth structure that in certain orientations can isolate carious lesions. The sensitivity of radiographic systems, however, is limited by visible changes in film density, making identification of small carious or precarious regions difficult. Since radiographs are two dimensional, precisely locating the position of such decay is impossible. Moreover, due to the orientation of the x-ray imaging, only interproximal lesions (between the teeth) are easily detected, while early occlusal lesions (top of the tooth), are difficult to detect. In addition, radiography uses harmful ionizing radiation.
Given the disadvantages of current detection techniques, a need exists for a device that provides safe, early diagnosis of caries. This invention applies the technique of polarimetry to image dental hard tissue and detect the presence of caries based on the depolarization of incident light. The invention also has the potential for detection of disease in bone.
Polarimetry is a well-established tool for non-invasive material characterization and involves comparison of the polarization states of light before and after the light interacts with the material. The use of polarized light for characterization and imaging of highly scattering media, such as biological tissue, has been studied. The effect of scattering on the polarization state of light has been found to be useful for imaging of surface or subsurface structures in scattering media, and for transmission imaging of deep structures. See Rowe et al., xe2x80x9cPolarization-difference imagingxe2x80x94a biologically inspired technique for observation through scattering mediaxe2x80x9d, Optics Letters 20:608-610 (1995). It has also been shown that the scattering parameters of turbid tissue, including the scattering coefficient xcexcsand anisotropy factor g, can be determined from diffusely scattered polarized light. See Hielscher et al., xe2x80x9cDiffuse backscattering Mueller matrices of highly scattering mediaxe2x80x9d, Optics Express 1:441-453 (1997).
Polarimetry may be combined with a second method, optical coherence domain reflectometry (OCDR), which was developed as a high resolution ranging technique for characterization of optical components and was based on bulk optics. See Youngquist et al., xe2x80x9cOptical coherence-domain reflectometry: a new optical evaluation techniquexe2x80x9d, Optics Letters 12(3):158-160 (1987). The first fiber optic based OCDR system was constructed by the U.S. National Bureau of Standards for micro-optic technology. See Danielson et al., xe2x80x9cGuided-wave reflectometry with micrometer resolutionxe2x80x9d, Applied Optics 26(14):2836-2842 (1987).
OCDR uses a low coherence Michelson interferometer to probe the sample, generating reflection signals as a function of depth. When the probe beam is transversed across the sample, a series of axial scans can be stacked together to form a high-resolution two-dimensional optical coherence tomogram. See Lee et. al, xe2x80x9cProfilometry with a coherence scanning microscopexe2x80x9d, Applied Optics 29(26):3784-3788 (1990). Optical coherence tomography (OCT) was developed to produce cross-sectional images of biological microstructure by combining transverse scanning with a fiber optic OCDR system. See Huang et al., xe2x80x9cOptical Coherence Tomographyxe2x80x9d, Science 254:1178-1181 (1991). U.S. Pat. No. 5,321,501 discloses the general means for construction of an OCT system, specifically as it applies to OCT imaging of the eye for diagnosis of ocular diseases. U.S. Pat. No. 5,459,570 discloses OCT imaging of biological tissue, including measurement of tissue optical properties and the use of polarization sensitive OCT (PS-OCT) to measure tissue birefringence. These OCT devices provide imaging in the eye and circulatory system.
PS-OCT has also been used for measuring birefringence in teeth in an unsuccessful attempt at caries detection. This attempt was unsuccessful because caries causes light to become depolarized by changing the scattering coefficient of the enamel rather than significantly affecting the birefringence of the enamel. See Baumgartner et al., xe2x80x9cOptical coherence tomography of dental structuresxe2x80x9d, Proc. SPIE 3248; Lasers in Dentistry IV, John D. Featherstone, Peter Rechmann, Daniel S. Fried, eds., pp. 130-136 (1998).
The application of OCT for dental applications was pioneered by the University of California at Lawrence Livermore National Laboratory. U.S. Pat. No. 5,570,182, assigned to the University of California, discloses the use of OCT for diagnosis of dental caries and periodontal diseases. Co-pending U.S. patent application Ser. No. 09/315,000 assigned to the same assignee, describes a dental explorer device for detecting caries and periodontal disease using OCDR, and is incorporated herein by reference. In order for OCT to be practical and convenient for clinicians to use on patients, an OCDR dental device was developed in the form of a hand-held, portable explorer tool for non-invasively probing teeth and other dental tissues. The OCDR explorer was designed to safely and accurately collect intraoral OCT images of dental tissue and microstructure in vivo for evaluation of dental health.
The capabilities of the dental explorer device have been further expanded and improved in the present invention by the incorporation of polarization sensitive diagnostics. The invention uses PS-OCT to measure the depolarization of light associated with optical scattering, rather than changes in polarization state associated with birefringence, to detect demineralization and caries. By taking advantage of the ability of polarimetry to both image tissue and detect changes in its scattering properties, a powerful diagnostic tool has been developed for detection of carious lesions.
This invention provides an optical technique and dental tool that use polarized light for early, non-invasive, and effective diagnosis of the state and structure of hard biological tissues (e.g., teeth and bone). The invention is particularly suited for detection of precarious and carious lesions, and may be useful for evaluation of dental restorations. The method is based on optically detecting the change in polarization of the incident light backscattered from dental tissues. In particular, the demineralization of tooth enamel that is the precursor to caries disease modifies the scattering properties of the tissue, resulting in depolarization of the incident light, which is then detected by the optical imaging system.
In the present invention, the tooth (or other mineralized tissue) is irradiated with polarized light having a selected or known polarization state; either circular, linear, or elliptical. Light backscattered from the tooth is then analyzed using optical polarimetry to determine its degree of polarization. The tooth can also be irradiated with multiple polarization states sequentially to differentiate between changes in polarization state associated with birefringence and depolarization. Depth-resolved images of the demineralization of the enamel can be obtained based on either the temporal or spatial coherence of the incident light by using optical coherence domain reflectometry or confocal imaging, respectively, and incorporating polarimetry. Depth-resolved information about the depolarization is useful as it enables identification of subsurface precarious and carious lesions and minimizes the effects of fresnel reflections from the front surface of the tooth, which can make the results more difficult to interpret.
The polarization sensitive optical imaging system can incorporate a dental explorer tool, which contains one or more optical fibers that independently couple light from the optical imaging system to the tip of a dental probe. The probe is placed against the tooth (or hard tissue), and light from the fiber at the tip of the probe is directed into the enamel. The light reflected or backscattered from the tissue is then collected by an optical fiber and detected by the optical imaging system. In a preferred embodiment, the polarized light is delivered and collected using a polarization sensitive OCDR system, which provides a single point profile of optical scattering (and thus tissue microstructure) as a function of depth.
The OCDR system consists of a light source split by a beamsplitter or fiber optic coupler into a sample arm and reference arm. Reflected or backscattered light from the tissue is collected in the sample arm and detected by heterodyning with the light in the reference arm. Only the photons in the sample arm that have traveled the same optical path length as the photons in the reference arm (within the coherence length of the source) generate a heterodyne signal. Thus, by varying the path length of the reference beam and recording the amplitude of the heterodyne signal, the OCDR system measures the scattering coefficient of the tissue as a function of depth. By moving the dental probe transversely across the tissue, the clinician can obtain a series of profiles of tissue microstructure. These profiles are combined to form a cross-sectional, or optical coherence tomography (OCT), image of the region of interest in the oral cavity. The polarization sensitive OCDR/OCT systems provide images of polarization state as a function of depth.
The object of this invention is to provide a dental tool that uses the depolarization of incident light to detect changes in the tissue microstructure that are indicative of disease. Another object of the invention is to provide a dental tool that combines polarimetry and optical coherence domain reflectometry. It is further an object of this invention to combine polarimetry with optical coherence tomography to generate depth-resolved images of the degree of polarization of light passing through tissue as a function of tissue depth. The invention uses PS-OCT to measure the depolarization of light associated with optical scattering, rather than effects due to birefringence. This invention is particularly suited to detecting the demineralization of teeth associated with caries, and may be useful for detecting demineralization of other mineralized tissues, such as bone. Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings.